[I'm 100% sure this is a question that's been asked and answered countless times before - and for that I apologize. I kept trying to winnow down a search but couldn't make it work.]

Anywhoo-

I'm on my first tile project in at least 3 years.
We need to re-do a 150 sq. foot bathroom floor that was installed directly on plywood 10 years ago. Lots of cracks and adhesion failures. Big surprise, eh? We got the tiles up with remarkable (maybe predictable!) ease, but the thinset stuck to the plywood quite a bit better than it did to the tiles.

So whaddya say:
-Grinding cup on a 4" angle grinder?
-Sander with a heavy grit?
-Something big and loud that scrapes?
-A week with a hammer and chisel?

I had good success on a similar job with a big floor scraper--you know, it's got a broom sized handle and a 1 foot blade at the bottom. I recall really bashing the thinset and tile with a sledge hammer and occasionally a cold chisel to break up the thinset and make it scrape off easier. There were still a few chunks left on the floor that I used a sander (and respirator) to remove. I had a lot of thinset that stuck to the tile and less to the floor, though, so maybe I was more lucky than you. In any case, one certainly learns that tile laid directly on plywood is asking for problems.

It's worth trying a scraper to see if it works--much less dusty than grinding/sanding the whole thing. Frustrating when you hit a nail head, though, but encourages you to reset any outstanding heads and locate any places that need some extra deck screws to secure the subfloor as you go along.

Cut and installed my first Ditra yesterday. Looks sharp except for a 3 foot long section where it's not well-butted together. I'm not sure why I didn't notice the sloppiness while it was still wet, but the cut is wavy and the gap is 3/8 at its widest - for about a foot, then tapers back to tight on either side.
Should I try to cut a sliver of scrap to fill it?
Fill it with thinset?
Something else?

Are you installing small format tile on the floor? If so you want to skim over your Ditra layer a day or two before setting. If you are setting anything bigger than a 6"x6" just leaving that floor as is would be OK.

Thanks gents - especially to whomever merged the threads - sorry for breaking protocol. (How do I re-name the thread? It's not giving me the option of editing the first post...)

Tiles are 13" x 13", with a 4 or 6 6"ers placed for variety.

To be honest, I had not planned to waterproof the project. I used Ditra primarily for its thinness (is that a word?). Now that I'm typing this, I realize that's probably fairly foolish given how easy it would be to Kerdi the seams.

If you want to waterproof the kerdi-band will have to be applied up the wall as well. In my way of thinking unless there is a way to contain the water then waterproofing with kerdi or kerdi-band is a waste of time and money.

A moderator can change the title of your thread. What do you want it to be?

I think we got sidetracked on the Ditra seam issue. The problem was a gap between two pieces of Ditra. There is no waterproofing issue that I could see. Question was if the gap was detrimental, and what needed to be done to correct it. I think the answer is to ignore that little uh-oh and promise to do better the next time.